Host processors or "mainframes" are often interconnected with a plurality of display terminals that enable user interaction with the host. The display terminals are typically found remotely located and are connectable with the host via the dial-up telephone network. Standard voice-grade telephone lines support asynchronous communications using ASCII character sets to encode the data. These lines typically transfer data at 1200-2400 bits per second. Certain modem makers are now marketing 9,600 and even 19,200 bits per second modems which provide considerably enhanced data transmission speeds; however, due to the lack of standardization and little movement by host manufacturers to provide high speed asynchronous access to their mainframes, many display terminals are still interconnected in the low-speed regime i.e. 1200-2400 bits per second. At such speeds, 10 to 20 seconds is required to change a display screen image, depending upon the host's workload and intermediate loads on the public data network. This problem is not present in local area networks, as data throughput therein is in the megabyte range and screen update is accordingly faster.
Previously, efforts have been made to increase data transmission speeds between a host computer and display terminals so that the inherent delays which result from the use of low-speed modems and switched networks are minimized. In U.S. Pat. No. 4,750,137 to Harper et al, a method, sometimes called the "delta" method, is disclosed for reducing the amount of data transferred between a host and an interconnected display terminal to increase the speed of updating of the display. The host retains an image of the present display and compares that image with a new display to be transferred to the terminal. Subsequent to the comparison, the host configures a data train which only contains indications of the differences between the present and new screens. This optimization is also extended to handling field attribute bytes which, for example, instruct the terminal how certain characters should be shown, i.e., bright, half-bright, flashing, etc. Attribute bytes may also permit or prevent the modification of certain fields on the display.
Others have provided similar types of transmission improvement protocols. The IBM Corporation has, for a number of years, provided a computer program known as the "Interactive System Productivity Facilities/Program Development Facility". That program compares, on a field-by-field basis, a memory image of the expected state of a remote terminal screen prior to an updating transmission, with a similar image of the state of the screen expected to occur following the updating transmission. It appears that this comparison is performed only if field boundaries have not changed. Any field in the updated-state image that exactly matches the corresponding filed in the present-state image is not transmitted. It also appears that in any field wherein an exact match does not exist, all characters in that field before and after the mismatch, are transmitted regardless of whether the mismatch continues to other characters in the field.
In Canadian Patent 1,193,745 to Bernstein, still another fast response transmission technique is disclosed. That patent describes a method for creating an illusion of increased retrieval speed of data from a storage disk. The Bernstein method describes a program which directs a communication controller to format information responses from a hard disk into two-message blocks, the first block being a short, fixed length that provides a fast response to a user's inquiry and the other block comprising the balance of the information response. The first sub-message is just long enough to fill one screen and the second sub-message may be the balance of a multiscreen image. The arrangement provides information which is immediately accessible to the user and subsequently provides additional information, while the user is engaged in reading the first screen.
Of the prior art known to Applicant, the "delta" process described by Harper et al is an effective technique for increasing data transfer between a host and remotely located display terminals. Nevertheless, it suffers from a number of drawbacks in actual use. For instance, so long as the currently displayed screen only differs from the new screen by a small amount, the delta corrections can be made rapidly and the data transferred efficiently. On the other hand, where a simple screen action such as a scroll occurs, then there is a substantial lack of identity between the current screen and the new screen and time is lost in the delta comparison process. Likewise, if there is a complete change of screen to another previously used screen, again the delta process bogs down as the comparison between the new and current screens is an essentially useless effort due to the screens being entirely different in character and content.
Accordingly, it is an object of this invention to provide an improved method for transfer of blocks of data from a host processor to another processor.
It is another object of this invention to provide an improved method for block data transfers between processors wherein delta comparison between new and old blocks of data is substantially enhanced by preprocessing.
It is a further object of this invention to provide an improved method of screen transfer between a host computer and a remotely located display terminal, wherein the display terminal is enabled to present a new screen in extremely rapid fashion.